The renin-angiotensin system (RAS) is a key regulator of blood pressure and fluid homeostasis. Within different human populations, the relative contribution of the RAS to the pathogenesis of hypertension and end- organ injury may vary widely. We hypothesize that some of this variation may be explained by genetic factors that modulate the functions of the RAS in health and disease; these factors may involve genes encoding the RAS components or other genes outside of the RAS. In our preliminary studies, we have identified genetic modifiers that dramatically alter kidney structure of mice with targeted disruption of the AT1A angiotensin receptor gene (Agtr1a). The objective of these studies is to use this mouse model to localize and characterize these naturally occurring genetic modifiers of the RAS through the following specific aims: Specific Aim #1: To define the influence of modifier loci on renal vascular phenotype in AT1A receptor-deficiency using inter-crosses and back crosses. In our preliminary studies, we have found that AT1A receptor-deficiency on inbred C57BL/6 or 129/J backgrounds has a severe renal phenotype, while kidneys are normal in F1(C57BL/6 x 129) AT1A receptor-deficient mice. We will perform F1 inter-crosses along with back crosses to the parental strains and analyze the progeny for the presence or absence of renal vascular lesions to further characterize these genetic modifiers. Precise definition of this phenotype in a large number of animals will be critical to the success of the mapping studies proposed in Specific Aim 2. Specific Aim #2: Mapping and identification of the modifying loci for renal vascular lesions in AT1A receptor-deficient mice. After a careful analysis of phenotype of progeny from back crosses and intercrosses, we will obtain genomic DNA and perform genomic micro-satellite analysis using markers that are polymorphic between the parental (C57BL/6 and 129) strains. Based on the results of this genome- wide scan, we will use linkage analysis to map the loci. Then, we will use a systematic approach to narrow the linkage intervals to attempt to isolate the modifier genes by positional cloning and candidate gene analysis. These studies provide the potential for identifying novel genes that have significant modifying effects on the physiology of the RAS. The combination of experimental methods that will be utilized provides an outstanding training vehicle for Dr. Le. Through the course of these studies, she should become fluent in a range of molecular genetic techniques that should provide a foundation for her career.